The present invention is directed to a method for reducing what are referred to as blocking artifacts with "discrete cosine transformation" (DCT) at a low data rate.
Discrete cosine transformation is employed in the coding event in a specific type of video scene coder. The image region is divided into adjacent, non-overlapping sub-regions, namely into "blocks" in which discrete cosine transformation is respectively employed. In coders for a low data rate, for example 64 kbit/s, various methods are employed in order to limit the rate in the course of the coding. Included among typical procedures is to limit the plurality of coefficient in the DCT transformation region for the individual blocks, i.e. to under-take a "frequency limitation". What are referred to as blocking artifacts can occur in the pictures as a consequence of these frequency limitations. The block structure thereby becomes a visual disturbance.
A filtering of the images in the proximity of the block boundaries with a smoothing filter has already been proposed, see, for example, H. C. Reeve III, Jae S. Lim, "Reduction of Blocking Effect in Image Coding", in Prc. ICASSP 1983 (Boston, Mass.), pages 1212 through 1215. A disadvantage of this filtering is that there is a visible unsharpening of the images in these regions.
The publication of B. Ramamurthi, A. Gersho, "Nonlinear Space-Variant Postprocessing of Block Coded Images", IEEE Trans. Acoust., Speech Signal Processing, Vol. ASSP-34, pages 1258 through 1268 has proposed a relatively complicated, unspecific postprocessing of the images. No information deriving from the coding event such as, for example, the frequency limitations, is exploited.